Hot-melt adhesives are used in the automated packaging industry for sealing cases and cartons. Usually, melted adhesive is extruded under high pressure through a nozzle, the adhesive being applied to upper and lower major and minor flaps of cartons in long continuous strips. The use of high pressure to force hot-melt adhesives through nozzle orifices presents an occupational risk, since a rupture in the equipment could spray hot material in any direction. Further, the expense of pressure resistant hoses, fittings and couplings could be eliminated if nozzle performance, using low pressure apparatus, could equal that produced with high pressure equipment. Much attention has been devoted to improving dispensing nozzles so as to provide adequate adhesive flow at lower pressures, as well as eliminating tailing, stringing, drooling, dripping and clogging between applications.
It has been realized that continuous strips of hot-melt adhesive represent a considerable use of material. One solution is to produce a series of short dots instead of a continuous strip. In U.S. Pat. No. 3,348,520, Lockwood discloses an apparatus which produces dots of hot-melt by opening and closing valves in the nozzles at a high cycling rate. Valves in the dispensing head are responsive to the alternating high pressure stroke and suction stroke of a pump. The valve in each nozzle also ensures clean sharp closure of the nozzles, thereby preventing any tendency towards dripping. Typically, cartons in an automated assembly line travel past the hot-melt adhesive dispenser at a rate of about 400 to 600 feet per minute. The rapid and repeated opening and closing of the valves, which is required to produce short adhesive dots, is hard on the seats and valves.
In U.S. Pat. No. 4,031,854, Sprague, Jr. teaches a method in which adhesive is extruded as a band of overlapping loops. A jet providing a gas stream has a rotational component causing swirling of the extruded adhesive filament. The gas stream should be heated to about 100.degree. F., the nozzle should be within three inches from the application surface, and the supply rate of fluid adhesive should be such that the filaments are at least two mil in diameter. Otherwise, the adhesive may harden, either before it reaches the application surface, causing stringing, or before the surfaces to be adhered are pressed together.
In U.S. Pat. No. 4,065,057, Durmann teaches an apparatus for spraying powdered heat responsive material, such as resin. The material is then melted by heated compressed air downstream of and away from the nozzle. This approach prevents problems, such as powder clumping, associated with apparatus which heat the material prior to being discharged from the nozzle.
Nozzle assemblies which spray melt materials have had limited success. Prior art units need to be about six inches from the application surface for proper spray formation, especially when the generally viscous hot-melt adhesives are used. However, at this distance the melt materials may cool and harden in ambient air before reaching the application surface. At low pressures, inadequate flow and improper spray formation, including misting, may occur. Misting, i.e., the production of extremely fine droplets of melt material, is undesirable for some applications, such as the sealing of cartons.
It is an object of the present invention to produce dispensing apparatus for hot-melt material which operates at low pressures and which results in a considerable saving of the amount of hot-melt material used.
It is another object of the present invention to produce a dispenser, especially for sealing boxes, which can spray preheated hot-melt adhesive onto a surface without stringing, misting or premature hardening of the material.